1. Field of the Invention
The invention relates to an optical scanning unit for scanning an information track on an optically readable record carrier, during which scanning the orientation of the scanning unit changes with respect to the direction of the instantaneously scanned track portion. Such scanning unit includes a radiation source, a radiation-sensitive detection system and an optical system for focusing a scanning beam supplied by the radiation source to form a scanning spot on the record carrier and for imaging the reflected scanning spot on the radiation-sensitive detection system. The detection system comprises a plurality of radiation-sensitive detection elements situated on either side of a bounding line and each supplying an electric output signal which is dependent on the intensity of the radiation incident on the detection element, the difference between the output signals of the detection elements on either side of the bounding line being representative of the magnitude and the direction of a deviation between the center of the scanning spot and the center line of the information track. The invention also relates to an apparatus for reading and/or writing information in an optically readable record carrier.
2. Description of the Related Art
A scanning unit of this type and a read and/or write apparatus of this type are known from U.S. Pat. No. 4,533,826 (PHN 10361). The scanning unit described therein comprises a focusing error detection system combined with a tracking system for reading an optical record carrier.
In such known unit it is ensured that the intensity distribution of the scanning spot image formed in the plane of the radiation-sensitive detection system will be symmetrical with respect to two detectors, if the center of the scanning spot in the information plane is situated on the center line of the scanned track. If the scanning spot moves transversely to the track direction, the intensity distribution on the two detectors changes, producing a difference between their output signals. Consequently, by subtracting the output signals from each other a difference signal is obtained which represents the magnitude and the sign of the tracking error and which can be used to correct the position of the scanning unit and hence the scanning spot with respect to the scanned track.
The information surface may have an information structure consisting of a single continuous track in the form of, for example, a spiral on a disc-shaped record carrier, or a plurality of successive tracks which can be successively scanned by the scanning unit and are, for example, concentric circular tracks on such a record carrier.
To enable the scanning spot to scan the entire spiral track or all concentric tracks, it must be possible to move the spot transversely to the track direction, i.e. in the radial direction in the case of a disc-shaped record carrier. If the optical scanning unit has a sufficiently light construction, that is to say, the assembly of elements for forming a scanning spot on the information surface and converting the radiation reflected from the information surface into electric signals, this movement of the scanning spot can be realized by moving the entire optical scanning unit in the radial direction. However, driving means actually used in practice for the optical scanning unit is a pivotal arm which has the scanning unit secured to its end, so that the scanning unit follows a circular path which crosses the track in the information surface at a constantly different angle. Such movement of the pivotal arm causes the bounding line between the detection elements which generate the tracking error signal to rotate with respect to the track direction or, in other words, the effective track direction will extend, for example, at an increasing or decreasing angle to the bounding line as the scanning unit moves across the information surface. The effective track direction is the direction of the projection of the instantaneously scanned track portion in the plane of the detection system. The result of the increase of the angle between the effective track direction and the bounding line is that the difference between the output signals of the detection elements on either side of the bounding line, which difference is associated with a given tracking error, becomes smaller. Due to the tracking error signal becoming smaller, the radial position of the scanning spot is corrected less accurately, and moreover the sensitivity to interference from scratches on the record carrier or other surface inaccuracies will increase.
The above-mentioned phenomenon may become a problem if, in order to achieve a further reduction in size of optical scanning devices, shorter pivotal arms are to be used resulting in an increase of the maximum angle between the effective track direction and the bounding line between the detection elements.